Question

How much energy is generated from freezing 2.5 g water

Answer 1

200 calories.
Explanation:
The (latent) heat of fusion of a material, is either one of:
1) the heat required to melt the material without
temperature change or
2) the heat removed from the material to freeze it
without temperature change.
For water this latent heat is 80 cal/g. Multiply this by
2.5 g to get 200 cal.